In recent years, due to the accelerating cost of fuels, it has become desirable to replace the conventional standing pilot used in gas furnaces with an interrupted type of ignition system. In the past the standing pilot has been the primary ignition source for gas furnaces. The standing pilot was very reliable and was very inexpensive to manufacture. In the days when gaseous fuels were relatively cheap, the continuously ignited standing pilot used an insignificant amount of fuel, from a cost standpoint. In recent years the shortage of fuels and the acceleration of their cost has made the standing pilot undesirable in certain types of applications. In addition, a number of states have legislated that installation of fuel burning equipment can no longer include a standing pilot in order to conserve fuels. This change in the status of the standing pilot has dictated that the standing pilot be replaced with some other type of ignition source.
The most common ignition source to replace the standing pilot has been a spark ignition source that typically uses a silicon controlled rectifier as the heart of a relaxation oscillator for the generation of an ignition spark. While this type of equipment is generally reliable, and only moderately more expensive than other approaches, it has the drawback of generating both audible and radio frequency noises. The constant arcing of a spark for generation of a flame at the pilot is objectionable. At the same time this arcing causes the generation of radio frequency noises that are transmitted in the normal power lines of a home and cause interference with other types of electrical equipment. For these reasons, the spark ignition systems that are replacing the standing pilot systems have deficiencies which make them of limited value.
An attempt has been made to provide other types of ignition systems for burners to replace the spark type of ignition systems. The most common replacement for the spark ignition system is a hot surface ignition system wherein an ignition element made of a high resistance metal or of a high resistance ceramic is used. The high resistance element is energized from a source of potential and will glow or be raised to an ignition temperature for the fuel being used. The drawback of this type of a system is that the hot surface ignitors have a relatively short life when used as an ignition element if kept constantly energized. As such, it has become necessary to provide a short energizing period for the hot surface ignitor, and then a means separate therefrom to monitor the existence of a flame, once one has been established.
In the earlier cross-referenced application which is assigned to the assignee of the present application, a system for energizing a hot surface ignitor and then monitoring the flame by a separate flame detection arrangement is disclosed. This arrangement has certain drawbacks in that a separate flame detection device is required in addition to the ignitor itself.